1. Field of the Invention
This invention relates to a system for the formation of a silicon thin film and a good-quality semiconductor-insulating film interface. Such silicon thin films are used for crystalline silicon thin film transistors, and such semiconductor-insulating film interfaces are employed for field effect transistors. The invention also relates to a semiconductor thin film forming system by the pulsed laser exposure method. Such semiconductors include, for example, silicon germanium (SiGe), silicon carbide (SiC), and other silicon compounds, and GaAs, GaN, CuInSe2, ZeSe, and other compound semiconductors. In addition, the invention relates to a system for the manufacture of driving elements or driving circuits composed of the semiconductor thin films or field effect thin film transistors for displays and sensors, for example.
2. Description of the Related Art
Typical processes for the formation of a thin film transistor (TFT) on a glass substrate are a hydrogenated amorphous silicon TFT process and a polycrystalline silicon TFT process. In the former process, the maximum temperature in a manufacture process is about 300° C., and the carrier mobility is about 1 cm2/Vsec. Such a hydrogenated amorphous silicon TFT formed by the former process is used as a switching transistor of each pixel in an active matrix (AM) liquid crystal display (LCD) and is driven by a driver integrated circuit (IC, an LSI formed on a single crystal silicon substrate) arranged on the periphery of a screen. Each of the pixels of this system includes an individual switching element TFT, and this system can yield a better image quality with less crosstalk than a passive matrix LCD. In such a passive matrix LCD, an electric signal for driving the liquid crystal is supplied from a peripheral driver circuit. In contrast, the latter polycrystalline silicon TFT process can yield a carrier mobility of 30 to 100 cm2/Vsec by, for example, employing a quartz substrate and performing a process at high temperatures of about 1000° C. as in the manufacture of LSIs. For example, when this process is applied to a liquid crystal display manufacture, such a high carrier mobility can yield a peripheral driver circuit on the same glass substrate concurrently with the formation of pixel TFTs for driving individual pixels. This process is therefore advantageous to minimize manufacture process costs and to downsize the resulting products. If the product should be miniaturized and should have a higher definition, a connection pitch between an AM-LCD substrate and a peripheral driver integrated circuit must be decreased. A conventional tab connection method or wire bonding method cannot significantly provide such a decreased connection pitch. However, if a process at high temperatures as in the above case is employed in the polycrystalline silicon TFT process, low softening point glasses cannot be employed. Such low softening point glasses can be employed in the hydrogenated amorphous silicon TFT process and are available at low costs. The process temperature in the polycrystalline silicon TFT process should be therefore decreased, and techniques for the formation of polycrystalline silicon films at low temperatures have been developed by utilizing a laser-induced crystallization technique.
Such a laser-induced crystallization is generally performed by a pulse laser irradiator having a configuration shown in FIG. 1. A laser light supplied from a pulse laser source 1101 reaches a silicon thin film 1107, a work, on a glass substrate 1108 via an optical path 1106. The optical path 1106 is specified by a group of optic devices including mirrors 1102, 1103, and 1105, and a beam homogenizer 1104. The beam homogenizer 1104 is arranged to uniformize spatial intensities of laser beams. Generally, the glass substrate on an X-Y stage 1109 is moved to irradiate a selected position on the substrate with a laser beam. The laser irradiation can be also performed by moving the optic device group or moving the optic device group and the stage in combination.
For example, J. Im and R. Sposili describe that a substrate is mounted on an X-stage, and a homogenizer is mounted on a Y-stage in FIG. 6 of “Crystalline Si films for integrated active-matrix-liquid-crystal displays”, Materials Research Society Bulletin, vol. 21, (1996), p. 39 (Reference 1).
Laser irradiation is also performed in vacuo or in a high purity gaseous atmosphere. Where necessary, the system has a cassette 1110 and a substrate traveling mechanism 1111. The cassette 1110 houses glass substrates each with a silicon thin film, and the substrate traveling mechanism 1111 serves to move the substrate between the cassette and the stage to house the substrate in the cassette or to mount the substrate on the stage.
Japanese Patent Publication (JP-B) No. 7-118443 discloses a technique of irradiating an amorphous silicon thin film on an amorphous substrate with a short wavelength pulse laser light. This technique can crystallize an amorphous silicon while keeping the overall substrate from high temperatures, and can produce semiconductor elements or semiconductor integrated circuits on large substrates available at low costs. Such large substrates are required in liquid crystal displays, and such substrate available at low costs may be glasses, for example. However, as is described in the above publication, the crystallization of an amorphous silicon thin film by action of a short wavelength laser light requires an irradiation intensity of about 50 to 500 mJ/cm2. However, the maximum emission output of a conventionally available pulse laser irradiator is at most about 1 J/pulse, and an area to be irradiated by a single irradiation is at most about 2 to 20 cm2, by a simple conversion. For example, if the overall of a 47 cm×37 cm substrate should be crystallized by action of laser, at least 87 to 870 points of the substrate must be irradiated with a laser light. Likewise, the number of points to be irradiated with a laser light increases with an increasing size of the substrate, for example, as in a 1 m×1 m substrate. Such a laser-induced crystallization is generally performed by a pulse laser irradiator having a configuration shown in FIG. 1.
To form uniform thin film semiconductor elements on a large substrate by the above technique, an effective process is known as disclosed in Japanese Unexamined Patent Publication (JP-A) No. 5-211167 (Japanese Patent Application No. 3-315863). The process includes the steps of dividing the elements to portions smaller than the beam size of the laser and repeating a combination of irradiation with several pulses and movement of the area to be irradiated by step-and-repeat drawing method. In the process, the lasing and the movement of a stage (i.e., the movement of a substrate or laser beam) are alternatively performed, as shown in FIG. 2B. However, even according to this process, the variation of lasing intensity exceeds ±5% to ±10% when the irradiation procedure is repeated at a density of about 1 pulse per irradiated portion to 20 pulses per irradiated portion using a currently available pulse laser irradiator with a uniformity of lasing intensity of ±5% to ±10% (in continuous lasing). The resulting polycrystalline silicon thin film and polycrystalline silicon thin film transistor cannot therefore have satisfactorily uniform characteristics. Particularly, the generation of a strong or weak light caused by an unstable discharge at early stages of lasing significantly invites such heterogeneous characteristics. This phenomenon is called spiking. As a possible solution to the spiking, a process of controlling an applied voltage in a subsequent lasing with reference to the results of integrated strengths can be employed. However, according to this process, a rather weak light is oscillated even though the formation of spiking is inhibited. Specifically, when irradiation periods and non-lasing periods alternatively succeed, the intensity of a first irradiated pulse in each irradiation period is most unstable and is varied, as shown in FIG. 3. In addition, the history of irradiation intensity differs from point to point to be irradiated. The resulting transistor element and thin film integrated circuit cannot have a significant uniformity in the substrate plane.
To avoid such a spiking, a process is known to start lasing prior to the initiation of irradiation to an area for the formation of element, as shown in FIG. 2A. However, this technique cannot be applied to a process of intermittently repeating the lasing and the movement of stage. To avoid these problems, a process is proposed in Japanese Unexamined Patent Publication (JP-A) No. 5-90191. The process includes the steps of allowing a pulse laser source to continuously oscillate and inhibiting irradiation of a substrate with the laser light by an optic shielding system during the movement of the stage. Specifically, as shown in FIG. 2C, a laser is continuously oscillated at a predetermined frequency, and the movement of stage to a target irradiation position is brought into synchronism with the shielding of an optic path. By this configuration, a laser beam with a stable intensity can be applied to a target irradiation position. However, although this process can stably irradiate the substrate with a laser beam, the process also yields increased excess lasing that does not serve to the formation of a polycrystalline silicon thin film. The productivity is decreased from the viewpoint of the life of an expensive laser source and an excited gas, and the production efficiency of the polycrystalline silicon thin film is deteriorated with respect to power required for lasing. The production costs are therefore increased. When a substrate to be exposed to laser is irradiated with an excessively strong light as compared with a target intensity, the substrate will be damaged. Such an excessively strong light is induced by an irregular irradiation intensity. In LCDs and other imaging devices, a light passing through the substrate scatters in an area where the substrate is damaged, and the quality of image is deteriorated.
A process for reducing and projecting a pattern on a photo mask onto a silicone thin film is disclosed by R. Sposili and J. Im in “Sequential lateral solidification of thin silicon films on SiO2”, Applied Physics Letters, vol. 69 (1996), p. 2864 (Reference 2), and by J. Im, R. Sposili, and M. Crowder in “Single-crystal Si films for thin film transistor devices”, Applied Physics Letters, vol. 70, (1997), p.3434 (Reference 3). The process disclosed in these publications performs an about 1:5 reduction projection alignment using a 308-nm excimer laser, a variable-energy attenuator, a variable-focus field lens, a patterned-mask, a two-element imaging lens, and a sub-micrometer-precision translation stage. By this configuration, the process attains a beam size and a travel pitch of a substrate stage, both of the order of micrometers. However, a laser beam applied onto the photo mask has a spatial intensity profile depending on the light source, and when the process is applied to the processing of a large substrate as mentioned above, the strength of a patterned light passing through the center of the mask and that passing through the periphery of the mask critically differ from each other. Accordingly, a crystalline silicon thin film having a desired uniformity cannot be significantly obtained. In addition, as an ultraviolet radiation with a short wavelength is reduced and projected, the focal depth of the beam is small and the irradiation depth is liable to shift due to warp or deformation of the substrate. With an increasing area of the substrate, the mechanical precision of the stage cannot be significantly ensured, and a little tilt of the stage or a displacement of the substrate on the stage disturbs a target laser irradiation.
A process is known for the laser irradiation. In this process, a plurality of pulses are applied while the irradiation of each pulse is retarded. This process is disclosed by Ryoichi Ishihara et al. in “Effects of light pulse duration on excimer laser crystallization characteristics of silicon thin films”, Japanese Journal of Applied Physics, vol. 34, No. 4A, (1995), p. 1759 (Reference 4). According to this reference, the crystallization solidification rate of a molten silicon in a laser recrystallization process is 1 m/sec or more. To achieve a satisfactory growth of crystals, the solidification rate must be reduced. By applying a second laser pulse immediately after the completion of solidification, the second irradiation of laser pulse can yield a recrystallization process with a reduced solidification rate. In viewing a temperature change (a time-hysteresis curve) of silicon as shown in FIG. 4, the temperature of silicon increases with the irradiation of laser energy, for example, as a pulse with an intensity shown in FIG. 5. When a starting material is an amorphous silicon (a-Si), the temperature further increases after the melting point of a-Si, and when the supplied energy becomes less than the energy required for increasing the temperature, the material begins to undergo cooling. At the solidifying point of a crystalline Si, the solidification proceeds for a solidification time and then completes, and the material is cooled to an atmospheric temperature. Provided that the solidification of silicon proceeds in a thickness direction from an interface between silicon and the substrate, an average solidification rate is calculated according to the following equation.Average solidification rate=(Thickness of silicon)/(Solidification time)
Specifically, if the thickness of silicon is constant, the solidification time is effectively prolonged to reduce the solidification rate. If the process maintains ideal conditions on thermal equilibrium, the solidification time can be prolonged by increasing an ideally supplied energy, i.e., a laser irradiation energy. However, as pointed out in the above reference, such an increased irradiation energy invites the resulting film to become amorphous or microcrystalline. In an actual melting and recrystallization process, the temperature does not change in an ideal manner as shown in FIG. 4, and the material undergoes overheating when heated and undergoes supercooling when cooled, and attains a stable condition. Particularly, when the cooling rate in cooling procedure is extremely large and the material undergoes an excessive supercooling, the material is not crystallized at around its solidification point, and becomes an amorphous solid due to quenching and rapid solidification. Under some conditions, thin films are converted not into an amorphous solid but into microcrystals, as shown in the above Reference 4. Such a microcrystalline thin film has an extremely small grain size as compared with a polycrystalline thin film or a single-crystal thin film. Thus, the microcrystalline thin film includes a multitude of grain boundaries each having a large grain boundary potential. If the thin film is applied to, for example, a thin film transistor, the resulting thin film transistor will have a decreased ON-state current or an increased OFF-state leak current.
Separately, processes are known, which include a step for the formation of a-Si thin film as a material to be irradiated with laser, a step for irradiating the thin film with a laser, a step for hydrogenation with plasma, and a step for the formation of a gate insulating film, in this order or in a modified order, while the material thin film is kept from exposure to the air. These processes are disclosed in the following publications.
Japanese Unexamined Patent Publication No. 5-182923 discloses a technique of subjecting an amorphous semiconductor thin film to a heat treatment and irradiating the treated thin film with a laser beam while keeping the thin film from exposure to the air.
Japanese Unexamined Patent Publication No. 7-99321 discloses a technique of moving a substrate having a laser-induced crystallized polycrystalline silicon thin film to a plasma-enhanced hydrogenation step and a formation step of a gate insulating film while keeping the substrate from exposure to the air.
Japanese Unexamined Patent Publication No. 9-7911 discloses a technique of moving a substrate having a laser-induced crystallized polycrystalline silicon thin film to a formation step of a gate insulating film while keeping the substrate from exposure to the air.
Japanese Unexamined Patent Publication No. 9-17729 discloses a technique of moving a substrate having a laser-induced crystallized polycrystalline silicon thin film to a formation step of a gate insulating film while keeping the substrate from exposure to the air. By this configuration, the surface of the polycrystalline silicon is kept from adhesion of impurities.
Japanese Unexamined Patent Publication No. 9-148246 discloses a technique of sequentially performing the formation of an amorphous silicon thin film, laser-induced crystallization, hydrogenation, and the formation of a gate insulating film, without exposing the work to the air.
Japanese Unexamined Patent Publication No. 10-116989 discloses a technique of sequentially performing the formation of an amorphous silicon thin film, laser-induced crystallization, hydrogenation, and the formation of a gate insulating film, without exposing the work to the air.
Japanese Unexamined Patent Publication No. 10-149984 discloses a technique of sequentially performing the formation of an amorphous silicon thin film, laser-induced crystallization, hydrogenation, and the formation of a gate insulating film, without exposing the work to the air.
Japanese Unexamined Patent Publication No. 11-17185 discloses a technique of sequentially performing the formation of an amorphous silicon thin film, laser-induced crystallization, the formation of a gate insulating film, and the formation of a gate electrode, without exposing the work to the air.
These concepts and techniques have been proposed to solve the following problems. Specifically, the surface of silicon formed by laser-induced crystallization is very active, and when the surface is exposed to the air, impurities are liable to adhere to the surface. Deteriorated or dispersed characteristics of the resulting TFT may therefore result.
Accordingly, the present inventors compared the performance between when an excimer laser-induced crystallization process and a silicon oxide film formation process are performed in the same system (including transfer of the substrate to another system without exposing the substrate to the air) and when the film is once exposed to the air. The results of this experiment revealed that the former technique can inhibit adhesion of dusts and particles and therefore greatly effectively improves yields of products. However, by increasing levels of cleanliness of clean room surroundings, equivalent advantages as above can be obtained to some extent. To improve the yields, a system including a film forming system and a cleaning mechanism of the substrate in the same system is most effective. This is because particles are adhered to the substrate during film-formation under some conditions in an a-Si film forming step, and the film must be exposed to the air to thereby be cleaned outside the system.
In contrast, differences in production processes do not significantly affect the performances of thin film transistors. The reasons for this may be supposed as follows. For example, K. Yuda et al. disclose a fixed oxide film charge density (1011 to 1012 cm−2) of a silicon oxide film and an interface state density (6×1010 cm−2eV−2 or less) between a silicon substrate and the silicon oxide film in “Improvement of structural and electrical properties in low-temperature gate-oxides for poly-Si TFTs by controlling O2/SiH4 ratios”, Digest of Technical Papers 1997 International Workshop on Active Matrix Liquid Crystal Displays, Sep. 11-12, 1997, Kogakuin Univ., Tokyo, Japan, 87 (Reference 5). The above silicon oxide film is formed at a temperature of about 300° C. to 350° C. with plasma or formed through a heat treatment at about 600° C. The silicon substrate is generally subjected to an “RCA cleaning”, is washed with water and is then introduced into a film forming system. In the RCA cleaning, the substrate is cleaned with an acidic solution, heated where necessary, such as a sulfuric acid-hydrogen peroxide mixture, a hydrochloric acid-hydrogen peroxide-water mixture, an ammonia-hydrogen peroxide-water mixture, or a hydrofluoric acid-water mixture. The aforementioned interface state density is obtained from a sample of a single-crystal silicon substrate that is exposed to the air after the formation of a clean surface (cleaning) and is then moved to the film-formation step.
Focusing attention to a trap state density of the laser-induced crystallized silicon film, H. Tanabe et al. disclose a trap state density of a crystallized silicon of 1012 to 1013 cm−2 in thin film transistors with laser-induced crystallized silicon films, in “Excimer laser crystallization of amorphous silicon films”, NEC Research and Development, vol. 35, (1994), 254 (Reference 6). These transistors exhibit satisfactory properties of a field effect mobility of 40 to 140 cm2/Vsec.
The trap state density of the silicon film is significantly larger than the interface state density (or fixed oxide film charge density) of the silicon film. Specifically, to obtain satisfactorily advantages of a clean surface of a sample that is obtained by forming a silicon film and a gate insulating film in the same system without exposing the substrate to the air, the performance (the trap state density) of the silicon film is still insufficient.
As a means for reducing damage by plasma and forming a gate insulating film of good quality, a remote plasma-enhanced chemical vapor deposition (CVD) process has been proposed. For example, Japanese Unexamined Patent Publication (JP-A) No. 5-21393 discloses a configuration in which a plasma generating chamber is separated from a substrate processing chamber. This configuration is supposed to attain such a low fixed oxide film charge density of 1011 to 1012 cm−2 and a low interface state density of 6×1010 cm−2eV−2 or less as mentioned above. However, this advantage is restricted by the performances of a silicon film which is previously formed.